Establish the Reason for Puppy Barking
To help you determine why your puppy is barking, you will first need to evaluate the circumstances that surround the behavior. If your puppy is barking at you, he is most likely trying to get your attention. A puppy that is standing over an item such as a bone or a food bowl may be exhibiting guarding behavior. Puppies that seem to be barking at "nothing" can simply be bored or frustrated. Many puppies become anxious when left alone, either in a crate at night, or when the pet parent is away from home.
Attention Seeking Barking
If your puppy is barking at you to get your attention, ignore her completely. Turn away from her, avoiding eye contact. Wait until the puppy stops barking, then give her the attention she is seeking. Be sure you do not speak to your puppy while she is barking at you. Do not give in if she continues to bark. Doing so will teach her that she will eventually get what she wants if she is persistent with the undesirable behavior.
Guarding
Puppies will sometimes guard a treasured item or their territory. To discourage this type of barking, you may teach your puppy that there is no need to guard by teaching him to "trade" for something of equal or greater value, such as a yummy treat in exchange for the shoe he is protecting. This will show your puppy that giving up one thing can lead to getting something even better.
Boredom
Making your puppy's environment more stimulating will usually resolve this type of barking. Giving him more exercise, rotating 4 or 5 different chew items and toys every few days, and feeding him from interactive dog toys instead of a bowl will keep him busy and quiet.
Home Alone Barking
If your puppy is barking while you are not home, you may wish to set up a video camera, webcam or ask for neighbor's observation to help you figure out if your puppy is barking because he is anxious about being alone, bored, or if there is something in his environment that is disturbing him such as other dogs barking, noisy vehicles or construction going on near the home.
Keep your puppy inside when you cannot supervise him if he is barking at passersby or noises. If he is bored, enrich his environment and provide him with more interaction when you are home.
Sometimes puppies are frightened by being left alone. If this is the problem, you may wish to look into a puppy daycare or seek the help of a professional trainer who can instruct you about the protocols necessary to resolve separation anxiety.